1. Field of the Invention
The present invention relates to an image display optical system which guides a luminous flux from a display device to an eye of an observer, and more particularly, to an optical system preferable for use in a head mounted display which is put on the head of an observer to enlargingly present an original image displayed on a display device to the observer.
2. Description of Related Art
Image display apparatuses of a head mount type (head mounted displays) have conventionally been known in which an image display device such as a CRT and an LCD is used and an original image displayed on the display device is enlargingly presented to an observer through a predetermined optical system.
Since such a head mounted display is put on the head of an observer, reductions in size and weight of the entire display are particularly needed. In addition, to produce a dynamic effect in a displayed image, an optical system used in the head mounted display needs to enlarge an original image displayed on the display device as much as possible for display at a wide angle of view and present the image at high resolution.
As a means for satisfying the needs, Japanese Patent Application Laid-Open No. H07 (1995)-333551, for example, has proposed the use of a decentered prism including a plurality of surfaces for an optical system which guides a luminous flux from a display device to an eye of an observer in which internal reflection in the decentered prism is used to fold an optical path to reduce the size of the optical system.
As a means for enlargingly displaying an original image displayed on a display device at a wide angle of view through an optical system which can be provided in a relatively short optical path, an intermediate image is effectively formed in the optical path between the display device and an eye of an observer. For example, Japanese Patent Application Laid-Open No. 2000-131614 has disclosed a head mounted display having an optical system which forms an intermediate image in an optical path.
When a head mounted display which achieves a small size and image display at a wide angle of view at the same time is to be formed by combining the aforementioned means, however, the following problem arises. Specifically, to form an intermediate image for display at a wide angle of view, it is necessary to provide a relay optical system for producing the intermediate image and an ocular optical system for presenting the intermediate image to an observer in a path between a display device and an eye of the observer. This complicates the required optical system to cause an increased size.
To solve the problem, an effective approach is to use a decentered prism to arrange the relay optical system and the ocular optical system in small space. In optical design using the decentered prism, a reduction in size is effectively possible by using a single surface of the prism a plurality of times in an optical path formed in the prism and increasing an optical power of each optical surface constituting the prism, although it is difficult to ensure optical performance as an image display apparatus and to suppress occurrence of various aberrations.